A typical seat belt comprises a length of belt webbing connected at three points to load bearing parts of a vehicle. Typically one end of the webbing is attached to a belt anchor that is bolted to a load bearing longitudinally extending chassis member on one side of a seat between the seat and the adjacent door. The webbing is arranged to pass laterally across the hips of a seat occupant to a buckle mechanism fixed to the vehicle on the opposite side of the seat, and then diagonally across the torso of the seat occupant, via a webbing guide, to a retractor mounted on the B pillar adjacent the door. A buckle mechanism engages a buckle tongue that is attached to the webbing in a sliding manner.
The retractor fitted at the pillar end of the webbing increases the comfort of the seat occupant restrained by the belt by allowing the webbing to pay out under relatively low loads to enable limited. This allows movement of the restrained seat occupant, for example to reach in-car entertainment controls or storage compartments. The retractor is biased to keep the webbing relatively taut about the seat occupant and a locking element is included to lock the retractor against webbing payout if an acceleration sensor senses the vehicle undergoing rapid acceleration or deceleration indicative of a crash.
In recent years, pretensioners have been introduced to rapidly pull in a length of webbing to tighten the belt about the vehicle occupant in a crash. This takes up any slack in the belt and helps to more correctly position the vehicle occupant in the seat to maximize the protection of the seat occupant provided by the seat belt and the protection of any secondary safety restraint such as an airbag.
Pretensioners use a force reservoir, such as a pyrotechnically operated gas generator, to provide an impulse of sufficient magnitude to tighten the belt in a short space of time, ideally before the crash pulse takes full effect.
Pretensioners are usually located at the retractor end of the webbing where they typically rewind the retractor mechanism to pull in a length of webbing. Pretensioners have also been used at the buckle fastening to pull back the buckle mechanism. Buckle pretensioners have the advantage that they effectively pull in twice as much webbing length for the same translational movement of the pretensioning mechanism because a single movement of the buckle will pull in both the lap and the shoulder portions of the webbing across a vehicle occupant at the same time. However there is limited space in a vehicle around a buckle fastening and there is a tendency in newer vehicles, particularly those with highly adjustable seats, for buckles to be smaller and buckle stalks to be shorter. This limits the performance of a buckle pretensioner.